DISCLAIMER: I can't provide a convincing proof that tmpfs is the right choice for this needs, yet. So please keep skeptical while reading this post.

The rationale

On debugging or testing with database, I think it would be a great idea to reduce the disk I/O operation by using random access memory, and tmpfs is the choice that come into my mind.

HOWTO

First, create a mount point for mysql and its temporary directory, I'll use /mysql and /mysqltemp respectively, and optionally append the following line at the end of file of /etc/fstab to automatically mount it:

tmpfs   /mysqltmp	tmpfs   rw,gid=mysql,uid=mysql,size=100M,mode=0750,noatime   0 0
tmpfs   /mysql		tmpfs   rw,gid=mysql,uid=mysql,size=15G,mode=0750,noatime   0 0

feel free to adjust the size, exec sudo mount or if you don't prefer adding new entry to fstab:

mount -t tmpfs /<mount-point> -o size=<size>

stop the mariadb service and edit /etc/my.cnf:

[mysqld]
datadir=/mysql
tmpdir=/mysqltmp

and install the default config: mariadb-install-db --user=mysql --datadir=/mysql

and optionally copy the existing database while the service is not running: the default datadir location is /var/lib/mysql/.

if you don't want to reconfigure and just want fully migrate previous settings, follow this stackoverflow's answer.

finally, you can start the service again: systemctl start mariadb.

known problem - can't use existed tmpfs /tmp as datadir.